Expression of CD34 glycoprotein in the developing yolk sac and extraembryonic blood vessels. (A) Yolk sac from a 4-somite stage embryo: staining appears to be confined mainly to the endodermal layer (e); light staining (arrow) is present between two blood islands in the mesodermal layer. (B) Yolk sac from a 6-somite stage embryo: there is staining in the endodermal layer and in scattered cells around the periphery of the blood islands (arrows). (C) Yolk sac and allantois of an 8-somite embryo: CD34 is expressed in the endoderm (e) and around the periphery of the blood islands (arrows), but not in the hematopoietic cells nor in the allantoic mesenchyme (al) except for small blood vessels (open arrow). (D) Yolk sac from a 10-somite embryo: staining of the endothelium of the vitelline blood vessels (v) is comparable with that of the dorsal aorta (da); large cells (arrowed) within the vessels are also stained. (E) Yolk sac of an 18-somite stage day 9 embryo, showing less prominent staining in the vascular endothelium and a subset of strongly stained cells (arrows) within the lumina of the vessels. (F ) Day 10 yolk sac, showing CD34 expression on the endothelium of flattened vessels; large stained cells are absent. (G) Proximal portion of the vitelline artery, showing CD34⁺ sites of possible haematopoietic activity. (H) Umbilical artery, showing CD34⁺ hematopoietic cells (arrow) and a thickened, possibly haematopoietic region. Scale bars (A through F on F ) = 50 μm.